The present invention relates to a semiconductor device, and more particularly to a superjunction semiconductor device having alternating columns of p-type and n-type conductivity type material in the active and termination regions.
Typically, in vertically conducting semiconductor devices the electrodes are disposed on two opposing planes. When the vertical semiconductor device is turned on, drift current flows along the thickness (i.e., vertical direction) of the semiconductor device. When the device is turned off, depletion regions extend vertically. To realize high breakdown voltage for a vertical semiconductor device, a drift layer between the electrodes must be made from a high resistivity material and have a relatively large thickness. However, the high resistivity and the relatively large thickness of the drift layer increase the on-resistance of the device. A higher on-resistance adversely affects the performance of the device by increasing the conduction loss and lowering the switching speed. It is well known that on-resistance of a device rapidly increases in proportion to the 2.5th power of a breakdown voltage (B. Jayant Baliga, Power Semiconductor Devices, 1996, PWS Publishing Company, page 373).
One technique to overcome this problem has been to use a semiconductor device with a particular junction structure. Such semiconductor device includes alternating columns of opposite conductivity type material formed in a drift layer in the active region of the device. The alternating columns of opposite conductivity type material provide a current path when the device is turned on while it is depleted to withstand the reverse voltage when the device is turned off. A semiconductor device with alternating columns of opposite conductivity type material is hereinafter referred to as a “superjunction semiconductor device”.
For a superjunction semiconductor device, breakdown voltage of the device can be approximated by the product of the thickness of the drift layer and the threshold electric field. In particular, if the charge quantities in the alternately arranged columns of high concentration n-type and p-type material are in equilibrium with each other, the breakdown voltage becomes independent of the resistivity of the drift layer. For this reason, reducing the resistivity of the drift layer does not lead to a drop in breakdown voltage, thus realizing high breakdown voltage and low on-resistance at the same time.
Despite the above advantages, the superjunction semiconductor device has a drawback in that it is difficult to stably implement a termination region surrounding the active region. This is because the low resistivity of the drift layer (due to high impurity concentration) causes the lateral electric field distribution in the transition region from the active region to the termination region irregular, thus reducing the stability of the device. Furthermore, vertical electric field distribution must meet predetermined conditions for obtaining high breakdown voltage. If the vertical electric field distribution is ignored, the breakdown voltage in the termination region may be undesirably lower than in the active region.
Thus, there is a need for a superjunction semiconductor device wherein both the on-resistance and breakdown voltage are improved.